DESCRIPTION: This research will investigate the developmental onset and age related changes in the cognitive, behavioral, social and neuroanatomical correlates of the neuropsychiatric phenotype associated with the fragile X condition in young males. Molecular correlates of these changes will also be investigated. The study uses a multimethod, longitudinal sequential design for a 2 year evaluation of development in 48 fragile X males compared to 48 age and IQ matched males with developmental language delay (DLD) of unknown etiology. All subjects will be between 36 and 71 months old at the time of enrollment. Two follow up assessments of the child's cognitive, behavioral, and social functioning will occur at 12 and 24 month intervals. Neuroanatomical imaging (MRI) will occur with all subjects from both groups at the initial evaluation and a random subset of 24 subjects from each group at the 24 month follow up evaluation. A second contrast group of 30 young males with Duchenne muscular dystrophy (DMD; an X linked genetic disorder), enrolled at 60 to 95 months, will be used as a single evaluation outcome comparison group for the fragile X males. Males with DMD will be individually matched on IQ, developmental age, race and SES with a fragile X male. Developmental areas specifically addressed include: 1) cognition; 2) communication; 3) behaviors involving deficits in self regulation (e.g., stereotypies, hyperactivity); 4) social interaction; and 5) brain structures (e.g., caudate, hippocampus, cerebellar vermis). It is hypothesized that cognitive, communication, behavioral self regulation, and social interaction deficits will differ among the groups, will increase over time in the fragile X group but not in the comparison DLD group. It is expected that age related changes in neuroanatomical structures indicating ongoing, detrimental effects on the postnatal central nervous system of fragile X but not control males will be observed. This study will contribute to understanding the associated effects of a genetically determined developmental disability during an age period in which a high degree of developmental change is expected. It will also help identify the multiple biological and environmental influences on development in young, developmentally delayed children.